Classic horseshoe abscess.

Classic ‘horseshoe abscess’ (in this case, related to atypical mycobacteria) that has spread through communicating synovial pathways, which include the ulnar and radial bursae proximally and the flexor tendon sheaths of the thumb and little finger distally. The differential diagnosis will be discussed in subsequent sections.

A variety of infectious and noninfectious diseases involve the synovial sheaths of the hand and wrist. On the flexor side, these synovial sheaths consist of the two palmar bursae-the ulnar bursa and the radial bursa-and the digital flexor tendon sheaths. To properly characterize such diseases, the radiologist should be familiar with the relevant anatomy and its implications on patterns of disease progression.

A fat-suppressed T2-weighted axial image at the level of the distal forearm reveals distension of the palmar bursae by complex material consisting of numerous low signal foci against a background of fluid signal intensity. The smaller radial bursa (short arrows) surrounds the flexor pollicis longus tendon (star). The larger ulnar bursa (long arrows) envelops the flexor digitorum superficialis and profundus tendons (dotted line). The flexor carpi radialis (asterisk) and flexor carpi ulnaris (arrowhead) tendons are extra-bursal in location.

A fat-suppressed T1-weighted post-contrast axial image at the level of the metacarpal shafts reveals avid synovial enhancement of the ulnar (long arrows) and radial (short arrows) bursae. Note that the low signal foci seen in (A) do not enhance. Observe the two-tiered arrangement of the flexor digitorum profundus tendons (above dotted line) and flexor digitorum superficialis tendons (below dotted line).

A fat-suppressed T1-weighted post-contrast axial image at the level of the metacarpal heads showing extension of synovitis into the digital flexor tendon sheaths of the thumb (1) and little finger (5). There is no involvement of the flexor tendon sheaths of the index, long, and ring fingers (2, 3, 4).